It is known that phospholipids which possess a diacetylenic functional group in each of two fatty acid chains, e.g. 1, 2-bis (10,12-ticosadiynoyl)-sn-glycero-3-phosphocholine (DC.sub.23 PC), self-assemble to form a variety of microstructures under certain well defined conditions in agueous solutions.
Vesicles are spheroidal structures in which one or more lipid bilayers are wrapped in a lamellar fashion to enclose a volume of solution. Vesicles range in size from approximately 50 nanometers (a nanometer is a billionth of a meter) to 25 microns (a micron is a millionth of a meter). These structures are also formed by non-diacetylenic lipids. The vesicles used in this invention were prepared in the manner described in a monograph by Paul Yager and Paul E. Schoen in the "Journal of Molecular Crystals And Liguid Crystals", vol. 106, pp 371-381 (1984).
Tubules are hollow, cylindrical structures composed of up to approximately ten bilayers; characteristic diameters are 0.2 to 3.0 microns, wall thicknesses are approximately 5 to 50 nanometers. Tubule length is a controllable parameter, based on fabrication procedure. The aspect ratio (ratio of length to radius) is therefore also controllable.
Preparation of tubules for this invention was accomplished by two methods. The first method involved raising the temperature of an aqueous vesicular dispersion above the chain melting temperature, T.sub.m, of the diactylenic lipid (43.degree. C. for DC.sub.23 PC) followed by slow cooling to just below T.sub.m as described in "Polymer Preprints", by A. Sinqh and J. Schnur, vol. 26, pp 184-185, (1986), in the above cited "Journal of Molecular Crystals and Liquid Crystals" monograph, and in U.S. Pat. Application No. 852,596, which was filed on April 16, 1986 now abandoned on an invention by Paul Schoen, Paul Yager, and Joel Schnur in Lipid Tubules. The structures thus formed are referred to as "thermally-grown tubules". In the second procedure, a non-solvent such as water is added to a solution of the diacetylenic lipid in an organic solvents such as an alcohol, until the solubility of the material in the mixed medium is exceeded and precipitation occurs. Such structures are referred to as "solvent grown tubules".
Helices are spiral-shaped structures that are also produced by the tubule formation methods described above. The helical structures have a pitch of about one micron and are typically one or two bilayers thick. The diameters of the helices are comparable to those of the tubules.
The diacetylenic lipid microstructures are known to undergo free-radical topotactic polymerization upon exposure to photons or electrons of sufficient energy, such as can be produced by ultraviolet (UV) radiation, by X-rays, by gamma rays, and by an electron beam. The integrity of the resultant polymeric microstructures is improved over their monomeric analogues in regard to thermal, chemical, and mechanical stability. In one utilization of this invention, diacetylenic microstructures were polymerized as a dispersion in aqueous alcohol (ethanol/water=20/80 v/v) at 0.degree. C. by exposure to 1.33 Mev .sup.60 Co gamma radiation at dosages of 6 to 9 Mrad.
The lipid microstructures referred to above are electrical insulators and those microstructures have weak diamagnetic susceptibility--that is, the lipid microstructures can be aligned only by a strong (2.times.10.sup.4 gauss) magnetic field and have poor electrical conductivity.